1. Field of the Invention
The present invention relates to a control device for a hybrid electric vehicle, and in particular, to a control device for a hybrid electric vehicle arranged such that a driving force of an engine and a driving force of an electric motor can be respectively transmitted to driving wheels of the vehicle.
2. Description of the Related Art
Conventionally, a so-called parallel hybrid electric vehicle capable of transmitting a driving force of an engine and a driving force of a motor to driving wheels respectively, have been developed and are in practical use.
As an example of such a parallel hybrid electric vehicle, Unexamined Japanese Patent Publication No. 2004-162670 (hereinafter referred to as Patent Document 1) proposes a hybrid electric vehicle that is arranged so that a driving state can be switched between a state in which a clutch is engaged and driving forces can be transmitted to driving wheels from both an engine and an electric motor, and a state in which the clutch is disengaged and only the driving force from the electric motor can be transmitted to the driving wheels.
With the hybrid electric vehicle described in Patent Document 1, in the event that an output torque of the electric motor is limited by whatever reason and the output torque of the electric motor is insufficient with respect to a required torque necessary for traveling the vehicle, the engine is started up so that output torque may also be obtained from the engine in order to secure necessary driving force.
In addition, with the hybrid electric vehicle described in Patent Document 1, in the event that regenerative braking torque is limited during regenerative braking, revolution speed of the electric motor is increased by changing a gear ratio of a transmission. In this manner, the electric motor is protected by reducing the torque of the electric motor.
As described above, with the hybrid electric vehicle described in Patent Document 1, in the event that the torque of the electric motor is limited during driving of the vehicle and the torque of the electric motor is insufficient with respect to the required torque, the engine is started up in order to compensate for the deficiency. Therefore, the engine will be repetitively started-up and stopped, thereby giving rise to a problem in that vibration and noise are generated from such start-up and stopping of the engine. In addition, there is a problem in that a time lag occurs between a start-up of the engine and the generation of the required torque, which results in a temporary shortage of driving force. These problems cause a decline in the driving feeling of the hybrid electric vehicle described in Patent Document 1.
In particular, upon start-up of the vehicle, there is an increased likelihood that the electric motor will be unable to perform a smooth start-up by itself, and the decline of the driving feeling due to the above-mentioned deficiency in driving force will become prominent.